The chemical structure of xanthan is composed of a linear cellulosic (1→4)-β-D-glucose polymer with trisaccharide side chains composed of mannose, glucuronic acid and mannose, attached to alternate glucose residue in the backbone. (Milas and Rinaudo, Carbohydrate Research, 76, 189-196, 1979). Thus xanthan can be described as a branched chain polymer with a pentasaccharide repeat unit; normal xanthan typically has 2000-3000 pentasaccharide repeat units. The xanthan polymer is typically modified by acetylation and pyruvylation of the mannose residues.
The fermentation of carbohydrates to produce the biosynthetic water-soluble polysaccharide xanthan gum by the action of Xanthomonas bacteria is well known. The earliest work was conducted by the United States Department of Agriculture and is described in U.S. Pat. No. 3,000,790. Xanthomonas hydrophilic colloid (“xanthan”) is an exocellular heteropolysaccharide.
Xanthan is produced by aerobic submerged fermentation of a bacterium of the genus Xanthomonas. The fermentation medium typically contains carbohydrate (such as sugar), trace elements and other nutrients. Once fermentation is complete, the resulting fermentation broth (solution) is typically heat-treated. It is well established that heat treatment of xanthan fermentation broths and solutions leads to a conformational change of native xanthan at or above a transition temperature (TM) to produce a higher viscosity xanthan. Heat treatment also has the beneficial effect of destroying viable microorganisms and undesired enzyme activities in the xanthan. Following heat-treatment, the xanthan is recovered by alcohol precipitation. However, heat treatment of xanthan fermentation broths also has disadvantages, such as thermal degradation of the xanthan. Heating xanthan solutions or broths beyond TM or holding them at temperatures above TM for more than a few seconds leads to thermal degradation of the xanthan. Degradation of xanthan irreversibly reduces its viscosity. Accordingly, heat treatment is an important technique with which to control the quality and consistency of xanthan.
Xanthan quality is primarily determined by two viscosity tests: the Low Shear Rate Viscosity (“LSRV”) in tap water solutions and the Sea Water Viscosity (“SWV”) in high salt solutions. Pasteurization of xanthan fermentation broths at temperatures at or above TM has been found to yield xanthan of a higher viscosity as indicated by higher LSRV and SWV values.
Xanthan polymer is used in many contexts. Xanthan has a wide variety of industrial applications including use in oil well drilling muds, as a viscosity control additive in secondary recovery of petroleum by water flooding, as a thickener in foods, as a stabilizing agent, and as a emulsifying, suspending and sizing agent (Encyclopedia of Polymer Science and Engineering, 2nd Edition, Editors John Wiley & Sons, 901-918, 1989). Xanthan can also be used in cosmetic preparations, pharmaceutical vehicles and similar compositions.
There is a need in the art to produce a xanthan polymer with higher specific viscosity characteristics in the unpasteurized state. Such a higher specific viscosity xanthan polymer could provide more viscosity at equivalent xanthan concentrations, for example, for food, industrial, and oilfield applications.